yeah, i was intentionally avoiding those since op had asked about heroes. that whole abstract tier (Chaos, Order, In-Betweener, Love, Hate, Death, Eternity, Infinity, possibly the celestials) have infinite power and are totally unkillable.
There's a few villains I intentionally left out also- like Michael Korvac, Surtur, Tyrant, Ego the Living Planet, Shuma-Gorath, Abraxas...that are only really killable via plot devices.
If we stick to just earth based villains, theres a couple that are COMPLETELY broken but rarely show up, or are just nerfed by writers for no reason. Absorbing Man should never lose to anyone, ever. His secret weakess is his own idiocy. Graviton has full control over gravity, with no real limits. This dude rides asteroids for fun, throws skyscrapers around like they're nothing, and rearranged the continents into the shape of his own face just because he could. Maelstrom had total control over/ absorption of kinetic energy- so powerful he turned himself into an abstract being.
There's TONS of these dudes floating around that don't really get much use or HAVE to be nerfed because of how broken they are.